This is a good point.
For me, BOTH are important.
I want a good record sheet that makes sense and gives the mechs some interesting strengths and weaknesses, but I also LOVE it when the artist nails the design and comes up with a beautiful mech.
Likewise, I find some mechs to be almost unbearably hideous to look at. The original Kintaro art, for example is just AWFUL.
Duane Loose's art can sometimes be hit or miss (like the goofy legs on the Stalker). But he came up with a lot of really solid designs that have stood the test of time pretty well, and they're often very recognizable (even after the modern facelift), because his design elements were so good to begin with.
A lot of the artists who came after Duane delivered some of the most hideous mechs in the game, and most of the mechs from that timeframe look inferior to the 3025 mechs in my opinion (even if some of them are still cool).
Record sheets are important but without the art and fluff every record sheet for a 55 ton biped mech is a Shadow Hawk. It's art and fluff that make the Shadow Hawk different from the Wolverine and Griffin.
I think all artists have hits and misses. I also don't expect every mech, or other unit, to be beautiful. It doesn't happen in RL. Why should it in BT? Besides, sometimes ugly is good. If I like it, I'll use it. If not, I won't. And sometimes it's just one piece of artwork. Sometimes, one piece of art isn't good while another is.
It is nice when art, fluff, and stats all match. I like Quirks but there's some quirks I wish were actual equipment and that we had units that mounted them.